1. Technical Field
The invention relates to a method and a device for the surface machining of a structure such as a cellular structure having at least one row of substantially aligned and regularly spaced recesses issuing onto the surface to be machined.
More specifically, the invention relates to a method and device for machining such a structure in accordance with a trajectory strictly parallel to one or more rows of recesses issuing onto the surface, over a random depth less than the thickness of the structure and from said surface.
A preferred application of the invention relates to the surface machining of a board or plate of a cellular material having tubular cells such as a honeycomb material, prior to the placing of sealing panels or skins on the two faces of the plate in order to form a sandwich panel usable more particularly in the aeronautical industry.
2. Prior Art
In a certain number of industries, such as the aeronautical industry, structural elements are frequently produced in the form of sandwich panels, which may or may not be planar. Such panels are in particular characterized by a high rigidity or stiffness and a relatively low weight.
Sandwich panels frequently comprise a core formed from a cellular material having tubular cells and on either side of which are connected, generally by bonding two skins.
For various reasons (reducing weight, increasing the life, improving the performance characteristics of structures produced with these materials, safety, etc.), it can be of interest to interconnect the different recesses delimited by the cells. Thus, when the cellular material having tubular cells is integrated into a sandwich panel, it becomes possible to drain liquids which may enter the recesses under certain conditions or balance the pressure of the air contained in the recesses if the external pressure is modified.
In order to implement these interconnections, it is known to produce notches in the walls separating adjacent recesses, prior to covering said recesses with the skin placed on the surface of the structure. GB-A-1 353 468, U.S. Pat. Nos. 4,749,150 and 5,543,198 reveal the interest of such notches, which can be of varied shapes and sizes. However, they fail to disclose any means for solving the manufacturing problems which will now be described.
In general terms, cellular materials having tubular cells are provided with recesses in rows juxtaposed in accordance with a regular pattern. In the most frequent case, the materials are said to be honeycomb materials and the recesses have a hexagonal cross-section. The walls of the recesses are usually of aluminium or of an organic or inorganic, composite material.
When it is wished to link the recesses of a cellular material by machining notches on the surface thereof, it must be ensured that the notches are correctly positioned and have controlled shapes and sizes in order to avoid negative effects on the products. This means that each notch must only link two adjacent recesses and must not extend to the junctions separating several recesses.
However, in practice, the plates of cellular materials having tubular cells are not regular and suffer from arbitrary defects linked with their manufacture (defective stacking of the sheets, deformations during expansion, undulations of the recesses, etc.).
As the material is not held by the addition of skins to its two faces, it has a limited inherent rigidity and consequently easily deforms when stresses are applied to its surface.
Bearing in mind the limited rigidity of cellular materials and the irregular nature of their networks of recesses, the production of notches solely in the parts of the walls separating two adjacent recesses gives rise to problems which are difficult to solve in practice.
Thus, if notches are machined by machines such as conventional biaxial milling machines ensuring the relative displacement of the cutting tool and the cellular material plate in accordance with the two axes of the plane of said plate, it is not possible to ensure a precise positioning of the notches in the walls of the recesses. In particular, the deformation of the material and the defects of the network of recesses are such that certain of the notches are then made at undesired points of the structure such as the junctions separating several recesses or walls which are machined over at least part of their length. This is seriously prejudicial to the mechanical performance characteristics of the sandwich panel produced with the thus machined material.
At present, no device is known permitting the machining of notches on the surface of such a structure whilst ensuring a sufficiently precise positioning of the notches relative to the rows of recesses, in order to reliably ensure that certain of the notches are not produced at undesired locations liable to prejudice the mechanical performance characteristics of the product obtained.
In general terms, the invention relates to a method for the surface machining of a structure having at least one row of substantially aligned and regularly spaced recesses issuing onto said surface, in accordance with a trajectory perfectly parallel to the row of recesses, no matter what the alignment defects of said recesses and no matter what the deformations undergone by the structure during machining.
According to the invention, this result is obtained by means of a method for machining a structure having at least one row of substantially aligned and regularly spaced recesses issuing onto the surface to be machined, characterized in that machining takes place by a relative displacement between a machining head and the structure to be machined, whilst being successively supported in at least some of the recesses of said row of recesses, so as to machine the surface in accordance with a trajectory parallel to said row.
According to a preferred embodiment of the invention use is made of a machining head incorporating at least one rotary member provided on its periphery with protuberances able to be successively supported in at least certain of the recesses during said relative displacement.
Advantageously, the rotary member is then applied to the surface of the structure with a substantially constant and predetermined force. Said force is chosen as a function of the nature of the machined structure, in order to avoid the deformation thereof, particularly by crushing, whilst ensuring a good penetration of the protuberances into the recesses.
As a function of the particular case, use is made either of a rotary member, which rotates freely on itself during said relative displacement, or a motor-driven rotary member, which drives the machining head in order to create said relative displacement.
When the rotary member is not motor-driven, an overall, relative displacement takes place between a tool holder and the structure to be machined, in accordance with a mean, rectilinear direction close to said trajectory and the machining head is mounted on the tool holder with an adequate slackness in the direction substantially perpendicular to the trajectory so as to ensure that said head follows the aforementioned trajectory whilst being supported in the recesses.
Advantageously bearing takes place on at least two rows of recesses on either side of a cutting tool carried by the machining head.
A preferred application of the method relates to the surface machining of a cellular material plate.
The invention also relates to a device for the surface machining of a structure having at least one row of substantially aligned and regularly spaced recesses issuing onto the surface to be machined, characterized in that it comprises a machining head and guidance means carried by the machining head and able to successively bear in at least some of the recesses of said recess row, during a relative displacement between the machining head and the structure to be machined, so as to machine the surface in accordance with a trajectory parallel to said row of recesses.